Hello Craig,
Thank you for your response! For the first part, how exactly does this code look like if I'd use the JavaScript client action? I can't just copy paste the code.. I've tried something like below, but this doesn't work.
var scriptNode=document.createElement('script');
scriptNode.innerHtml = (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXXXX');
document.querySelector('head').appendChild(scriptNode);